Stephen King's 1986 novel is almost a meta-horror story, with frighteners drawn from the back catalogue of Hollywood ghouls (a werewolf, a mummy, Frankenstein's monster), nature gone mad (plagues of leeches and piranhas, a fish-man), and sci-fi weirdness (It is in fact a creature from outer space; the known universe was vomited up by a turtle called Mataurin). If it all sounds like the work of a drug-addled mind, that's because it was: King was in the grip of a raging addiction to cocaine and alcohol when he wrote It, later claiming to have been sober for no more than three hours a day at the time.

The 1990 television version of It - a two-parter, starring Tim Curry as the child-murdering clown - wisely left many of the more ridiculous aspects of the story out. The new film, which has Bill Skarsgard (son of Swedish acting legend Stellan) in the famous red fright wig, does likewise, but we'll have to reserve final judgement about what's been done with the trippier elements of King's story until we see Chapter 2 - told from the adult perspective - in 2019.

Skarsgard's Pennywise is a terrifying creation. So was Curry's, but the advances in special effects in the past 27 years ensure there is barely a trace of the more laughable aspects of the character this time around. Still, Pennywise is far from the only horror in the film.

People disappear or die in Derry, a small town in middle America, at a rate six times the national average, says chubby nerd Ben (Jeremy Ray Taylor), one of the teenage members of the Losers, the gang of outcasts that does battle with It. "And that's just grown-ups," he adds. "Kids are worse. Way, way worse."

The carnage comes in waves, every 27 years. Then, after 18 months or so, it stops. Until the next time.

But it's not just the deaths and disappearances that make Derry so dire. There's an epidemic of bullying, neglect and abuse - sexual, physical and psychological. All the while, the adults carry on as if nothing were amiss.

That's the real horror in this version of It: the cycle of abuse, and the forgetting or repression of any memory of that abuse, which allows for it to resurface and repeat as the victims become perpetrators in adulthood.

Pennywise is the manifestation of whatever it is that each of the kids fears most. But at a deeper level, he is also a manifestation of the damage done by shame, trauma and secrecy in the abuse of children. He could just as easily be dressed in a cassock as a clown suit, but we'd be in very different territory if he were.

It is in fact an incredibly dark and powerful allegory, hiding behind the trappings of a "mere" genre film about, and seemingly aimed at, teenagers. But then that's what the best horror movies do - promise cheap thrills while trawling the emotional and psychological depths.

A good horror movie leads the audience through the door of our well-lit conscious minds and into the dark basement where our deepest fears and anxieties lurk. Often, we don't even know what's down there until we stumble upon it.

Home invasion, rape, murder, child abduction, being wrongfully accused of a crime, falling victim to a sophisticated scam - these are the staple fears of the adult mind, and they lend themselves most readily to the thriller or suspense genres.

But the mind of a teenager or young adult is more beset by anxiety relating to group dynamics, to the body, to adults whose intentions are unclear, to sex. Especially sex. And this is where horror finds its most fertile ground.

Think of Carrie, a story about ostracism and menstrual anxiety (both themes that also surface in It). Think of It Follows, a film about promiscuity and disease (the afflicted can only free themselves by passing the curse on to another through sexual intercourse). Think Raw, in which a young woman's first experience of sex at university is intimately linked to a ravenous desire for human flesh. Think Alien, in which the very notion of reproduction is bound up with the horror of both penetration and bodily expulsion.

Like Pennywise himself, horror movies tap into our deepest fears and anxieties and give them shape and form. But those shapes and forms are themselves mutable, so that The Blob in the 1950s manifests an anxiety about the unforeseen consequences of the nascent area of space exploration, while Life (2017) updates that anxiety to reflect the fresh reality of real-world missions to the planets and asteroids of our solar system.

The zombie genre is especially malleable, serving as metaphor for the horrors of Vietnam in George Romero's Night of the Living Dead (1968) and of rampant consumerism in his Dawn of the Dead (1978), for the dangers of viral warfare in 28 Days Later (2002) and World War Z (2013), the collapse of civil society and government in The Walking Dead (2010-) and its spin-off Fear the Walking Dead (2015-).

Sexual abuse (Split), cyber bullying (Unfriended), grief and mental illness (The Babadook), the predatory nature of the entertainment and beauty industries (Neon Demon), and the racism lurking beneath the veneer of white liberalism (Get Out) have all surfaced in horror films released in the past year or so. These are rich pickings indeed, for young and older audiences alike.

None of this is to deny that people go to see horror movies because they want to experience the sheer visceral pleasure and pain of being scared or shocked or simply horrified by what's up there on screen. They like the experience of confronting fears and, usually, seeing the hero overcome them.

But it is to suggest that in the best of those movies there's more going on too. Lots more.

So the next time someone dismisses such an offering as being "just" a horror film, maybe they should be urged to look a little deeper.

If they dare, that is.

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